1. Field of the Invention
The field of the invention relates generally to an encapsulation structure for solid state motor actuators, and more particularly to a process for encapsulating piezoelectric solid state motor stacks.
2. Related Art
For decades electroexpansive materials have been employed in stacked structures for producing actuation used for fuel injection and valve control in diesel engines, for example. Commercially manufactured solid state motor stacks, or actuators, are produced using piezoelectric disks interleaved with metal foil electrodes. Application of high voltage, low current power to alternately biased electrodes causes each of the piezoelectric disks to expand or axially distort. The additive deflection of the stacked disks is typically amplified by hydraulics to effectuate useful actuation.
An example of a conventional electromechanical actuator having an active element of electroexpansive material is found in U.S. Pat. No. 3,501,099 to Glendon M. Benson. Benson's 1970 patent is directed to both an actuation amplification structure and a method for manufacturing piezoelectric stacks. Sheets of ceramic material are rolled, compacted and punched into ceramic disks. After a cleaning process, the disks are stacked with alternate sets of continuous disk electrodes disposed between the ceramic disks. The stacks undergo a pressurized cold-welding process, followed by an elevated temperature and pressure bonding process after common electrodes are connected to the two electrode groups. The stacks are poled by application of a DC voltage and then encapsulated with a plastic insulative cover prior to final mounting within a transducer housing.
Another conventional method uses epoxy to bond lead zirconium titanate (PZT) ceramic discs together, interleaved with metal foil electrodes. The epoxy used in this process has a lower modulus of elasticity than the ceramic and acts as a compliant layer in the bonded stack structure, thus reducing the additive displacement during actuation.
A modified technique for preparing piezoelectric disks includes a two-stage press/lap manufacturing process. After punching or individually compressing the disks, the thickness of each disk is reduced by lapping. Piezoelectric disk thicknesses are limited to approximately 0.254 mm by this process and require high voltage to generate lattice distortion. Higher driving voltages necessitate larger, more expensive power generation circuits. In turn, the delivery of the higher voltage to the stack imposes wiring harness and connector design difficulties. Moreover, the larger power circuits generate additional heat which must be removed from the engine. These tradeoffs plague stack design and drive up the cost of systems using stacks to actuate valves or injection controls, for example.
Moreover, the ability to mass produce stacks is limited by the time and cost of manufacturing the disks themselves. Conventional punching or compressing combined with lapping/polishing results in low disk yield due to the time elements of the two step process and disk breakage during the lapping step.
Various environmental design considerations are important in piezoelectric stack manufacturing. Device operating temperature ranges and external mechanical stresses are the most serious of these factors.
Conventional stacks are limited to a maximum operating temperature of about 75.degree. celsius, measured at the outside of the stack housing. Heat generated by the stack itself is compounded by the extreme heat generated by the engine upon which the housed stack is typically mounted. Stack temperatures can reach upward of 40.degree.-50.degree. C. above the measured engine temperature.
On the other hand, structural defects typically lead to conventional stack failure due to shear and torsional stresses applied to the stack during operation and/or installation. Structural stack failure is most commonly attributed to fatigue cracking of the ceramic disks. Separation between disks/electrodes is also a frequent problem.
Piezoelectric stack insulation has been introduced between the disk/electrode stack and the housing in an attempt to minimize some of the above mentioned problems.
U.S. Pat. No. 4,011,474 to Cormac G. O'Neill discloses several methods for improving stack insulation to avoid operation breakdowns. Arc-over is allegedly avoided by maintaining contact between the piezoelectric stack and the insulating material. In a first embodiment, O'Neill teaches introducing a pressurized insulating fluid such as oil, into the housing of a piezoelectric stack. The fluid is pressurized so as to maintain contact between the fluid and the stack during radial shrinkage, or axial expansion, upon the application of an applied voltage.
In a second embodiment, O'Neill applies a solid polyurethane coating to the stack. The coating is kept in contact with the stack by a pressurized insulating fluid to prevent separation during operation and arc-over associated therewith.
A third O'Neill embodiment maintains contact between the stack and a solid insulating coating by winding a filament or tape around the coated stack. The tape is wound around the coating to preload the coating to prevent separation of the coating from the stack. The winding of the tape is spaced to allow for expansion of the polyurethane coating during operation of the stack.
The present invention constitutes an improvement over conventional encapsulation technology. Benefits, such as increased stack operational temperature range, endurance, output, and lifetime, are achieved by the present invention.